Electrotherapy devices which can ascertain the contractility of a heart afford the possibility of adapting a therapy to be delivered by the electrotherapy device to the respective contractility state of the heart of the patient.
Contractility describes an inotropic state of the heart. It influences the force and the speed of a myocardiac contraction. Contractility is controlled by three mechanisms:                direct control by the autonomous nervous system (ANS),        the so-called Starling mechanism, and        the so-called Bowditch effect (force-frequency coupling).        
The main mechanism, control of circulatory regulation by the autonomous nervous system, increases contractility and heart rate when there is an increased metabolic demand, for example in the case of bodily or physical effort, in order to ensure a suitable blood supply.
In the case of patients with chronic heart failure (HF), myocardiac contractility decreases to a low level and interventricular synchronization is worsened. That is accompanied by a low ejection fraction (EF), as well as a low quality of life and a high mortality rate. HF occurs frequently in the population. HF patients are treated with various drugs which influence the inotropic state, for example betablockers, in order to stabilize the heart rate, but also with positive inotropic drugs, for example glycosides, in order to increase contractility. In more recent times HF patients are treated with resynchronization therapy devices, for example 3-chamber cardiac pacemakers or defibrillators. The aim of such pacemaker therapy is to synchronize the two ventricles of a heart by biventricular stimulation in order to improve the temporal performance of the chambers of the heart and thus heart efficiency (cardiac resynchronization therapy or CRT).
Contractility is therefore an important parameter to be observed, in particular for HF patients. Such observation is important in order                to observe the state of the patient and an alleviation or progress in the disease,        to establish and observe resynchronisation therapy of the heart (cardiac resynchronisation therapy or CRT), and        to observe drug treatment.        
Information about contractility can also be used to optimize cardiac pacemaker therapy or a therapy implemented by means of an implantable cardioverter/defibrillator (ICD).
Although the contractility is a parameter of great importance, it is difficult to measure in clinical practice. It is usual for contractility to be determined on the basis of a maximum ventricular pressure gradient dp/dtmax in the right ventricle or in the left ventricle. A left-ventricular ejection fraction can also be determined by means of electrocardiography. An investigation of the right ventricle by means of electrocardiography is very difficult for anatomical reasons, although the information about the right ventricle is highly important for a complete investigation. Both procedures, pressure measurement and electrocardiography, are time-consuming and expensive. Ventricular pressure measurement requires an invasive intervention. It requires a pressure catheter in one or both ventricles and can only be implemented during an electrophysiological study or implantation of a cardiac pacemaker or cardioverter/defibrillator.
Implants with which the contractility of a heart can be ascertained are described, for example, in U.S. Pat. Nos. 4,674,518 and 5,417,717. In those cases measurement of the change in the heart chamber volume is effected by means of the pressure gradient dP/dt and by means of impedance plethysmography.
European patent application No 1 062 974 describes a cardiac pacemaker of the kind set forth in the opening part of this specification. The aim is for the control method described therein for an electrotherapy device and the electrotherapy device disclosed therewith to be further developed to expand the range of use of the electrotherapy device.